1. Field of the Invention
The present invention relates to an optical pickup apparatus executing a reading-out operation of a signal recorded in an optical disc or a recording operation of a signal in the optical disc.
2. Description of the Related Art
An optical disc device capable of a reading-out operation and a recording operation of a signal by applying laser light emitted from an optical pickup apparatus to a signal recording layer of the optical disc has been in widespread use. While the optical disc devices using optical discs called CD or DVD are widely available, optical discs in which recording density is improved, that is, those using Blu-ray standard or HD DVD (High Definition Digital Versatile Disc) standard optical discs have recently been developed.
Infrared light with a wavelength of 780 nm is used as the laser light for executing the reading-out operation of a signal recorded in the optical disc of the CD standard, and red light with a wavelength of 650 nm is used as the laser light with which the reading-out operation of a signal recorded of the DVD standard optical disc is executed.
The thickness of a protective layer provided on a top face of the signal recording layer in the CD standard optical disc is 1.2 mm, and the numerical aperture of an objective lens used for the reading-out operation of a signal from this signal recording layer, is specified to be 0.45. The thickness of the protective layer provided on the top face of the signal recording layer in the DVD standard optical disc is 0.6 mm and the numerical aperture of the objective lens used for the reading-out operation of a signal from this signal recording layer is specified to be 0.6.
In contrast to the case of such CD-standard and DVD-standard optical discs, in the Blu-ray standard or HD DVD standard optical discs, a laser light with a short wavelength, for example, a blue-violet light with a wavelength of 405 nm is used as the laser light executing the reading-out operation of a signal recorded therein.
The thickness of the protective layer provided on the top face of the signal recording layer in the Blu-ray standard optical disc is 0.1 mm, and the numerical aperture of the objective lens used for the reading-out operation of a signal from this signal recording layer is specified to be 0.85.
For a reproduction operation of a signal recorded on the signal recording layer provided in the Blu-ray standard optical disc or for recording a signal on such a signal recording layer, a diameter of a laser spot formed by condensing laser light needs to be reduced. In the objective lens used for obtaining the laser spot shape with a small diameter, not only because the numerical aperture (NA) thereof is increased but also because the focal length thereof becomes shorter, a radius of curvature of the objective lens becomes smaller.
In the optical pickup apparatus, a laser diode is used as an element for generating and radiating the laser light, but intensity of the laser light to be emitted from the laser diode is not constant but has properties called Gaussian distribution. The intensity distribution of such a laser light is in an oval shape, which is well-known in the case of the laser diode.
In the optical pickup apparatus using the objective lens with a small radius of curvature, since the oval laser light emitted from the laser diode enters the objective lens, there decreases rim intensity, i.e. the intensity of the laser light passing through the objective lens in a peripheral portion thereof relative to the intensity of the laser light passing through the objective lens in the vicinity of an optical axis thereof. As a result, there is not only a problem that the peripheral portion of the laser spot becomes blurred but also a problem that focusing intensity decreases.
In such problems, a lens with a small radius of curvature and a large numerical aperture such as the objective lens as mentioned above causes an incident angle of the laser light to become larger on the peripheral side, i.e. an area of a large numerical aperture side, and thereby causes reflectance of the laser light to become larger. Such properties result in the decrease of the above-mentioned rim intensity.
A method of forming an antireflection coating on the objective lens is proposed as a method for solving such a problem and obtaining a desired laser spot (See Japanese Patent Laid-Open No. 2004-39161).
In the Japanese Patent Laid-Open No. 2004-39161, there is described an art in which an antireflection coating is provided on an objective lens so as to minimize the reflectance in a predetermined area by the antireflection coating. Even with this art, though the rim intensity can be improved, the optimal characteristics can not be obtained over the entire region of the objective lens.